Feedback in an audio amplifier occurs when the acoustic signal from the output transducer finds its way back to the input transducer of the amplifier, thus creating a feedback loop. In audio amplifiers such as hearing aids, feedback can result in audible whistling or howling.
In a hearing aid, feedback occurs when the sound delivered to the ear canal leeks back to the microphone input. There are many feedback paths for sound to take, the most significant of which is via an open vent in the ear mould, although other paths such as gaps between the ear mould of the hearing aid and the ear, do exist. When fitting a hearing aid with a very high gain, it would be desirable to completely block the vent to improve feedback problems due to the high gain. However, it is not practical to completely block the ear mould vent for several reasons. Blocking the vent completely causes ear occlusion resulting in changes to the sound of the wearer's own voice. Moreover, blocking the vent prevents air flow needed for hygiene and comfort of the wearer, and reduces the transmission of unaided low frequency sounds into the ear.
A theoretical model of a hearing aid system is shown in FIG. 1. In this Figure, H is the forward transfer function of the hearing aid amplifier, and G is the transfer function of all combined feedback paths. If there is a vent in the ear mould, the transfer function G is dominated by the feedback path via the open vent. Both transfer functions H and G are complex functions of frequency. In order to minimise the above described problems resulting from the feedback loop in the model shown in this figure, various types of feedback cancellation systems have been proposed.
Typical feedback cancellation systems are based on altering the gain or the sound signal over the range of frequencies where feedback occurs. However, reduction of gain over a wide range of frequencies is not advantageous if the amplifier does not achieve the desired output level. Using a feedback detection algorithm, narrow band high intensity sounds can be detected and interpreted as the onset of feedback oscillation. A tuneable notch filter can be used to reduce the gain over a narrow frequency range, centered on the detected frequency. Some feedback cancellation systems employ several tuneable notch filters in a situation where the closed loop gain becomes unstable at several frequencies simultaneously.
Another approach to feedback avoidance that has been used is the use of frequency translating amplifiers. A frequency translating amplifier is one which transposes the frequency of the input sound signal, either upward or downward, in addition to amplifying the signal before sending it to the output transducer. One such frequency translating amplifier is described in European patent application EP04/005270.6 entitled “Method for frequency transposition and use of the method in a hearing device and a communication device,” in the name of Phonak AG. The manner in which a frequency translating amplifier operates is illustrated by the model shown in FIG. 2.
In such a system, a frequency transposing component referenced T is added to the output of the forward path transfer function of the simple closed loop feedback system shown in FIG. 1. The frequency of the amplified external signal is translated to a different frequency. The receiver output, and hence the feedback signal, is now at a different frequency from that of the external input signal so that successive summation of a signal at the microphone input at a particular frequency cannot occur. As described in M. R. Schroeder, “Improvement of acoustic-feedback stability by frequency shifting,” J. Acoust. Soc. Am. 36, 1718-1724˜1964 the amount of frequency transposition required is very small, and may typically be in the order of 5 Hertz for a frequency transposition public address system.
Frequency translation makes an amplifier stable for the same gain that would otherwise cause instability, and hence howling, without frequency transposition. However, a frequency translating hearing aid may be stable in terms of its closed loop gain, but when the hearing aid forward gain is equal to or greater than the attenuation of the feedback path, unwanted artefacts are introduced which decrease the quality of the sound. One such method of reducing these artefacts is described in Australian patent application 2003236382 entitled “Feedback suppression in sound signal processing using frequency transposition,” and its corresponding U.S. patent application Ser. No. 10/921,550, both assigned to Phonak AG.
Another approach to feedback reduction in audio amplification devices that has been adopted is the phase inverting feedback canceller. These cancellers operate by taking the correlation between the actual microphone input signal and a previous output signal sent to the receiver, and cancelling the correlated component. However such systems cannot distinguish between correlations introduced by the source signal (e.g. vowels or tonal components in music) and correlations introduced by the feedback signal. As a result these devices are better described as being correlation-cancellers, in the sense that rather than acting only on feedback signals such systems effectively cancel any input signal that correlates with the receiver output.
Accordingly, it is an object of an embodiment of the present invention to provide a method of processing a sound signal that addresses at least one of drawbacks of the prior art.
It is an object of an embodiment of the present invention to provide a device or processing a sound signal that addresses at least one of drawbacks of the prior art.
The applicant does not concede that the prior art discussed herein forms part of the common general knowledge in the art at the priority date of the application.